1. Field of the Invention
This invention relates to a high pressure discharge lamp with a starting circuit contained within an outer envelope.
2. Description of the Prior Art
In a high pressure discharge lamp, especially a discharge lamp like a high pressure sodium vapor lamp having a fill of metallic sodium enveloped within an arc tube, the starting voltage is high, and usually ranges from 2 to 3 KV. The discharge lamp requiring such a high starting voltage cannot be turned on even if it is directly connected to a commercially available power source. In the past, therefore, a starter or a pulser has been added as an extraneous starting device of the discharge lamp to generate a pulse of higher voltage than the starting voltage of the discharge lamp so that such a pulse of high voltage may be applied to the discharge lamp during the starting. However, provision of such a separate starting device for generating a pulse of high voltage requires increased facilities and accordingly higher cost.
To overcome such problems, there has been proposed a discharge lamp which eliminates the extraneous starting device and has a starting circuit contained within the outer envelope or bulb of the discharge lamp. In the discharge lamp of this type, the starting circuit has comprised a combination of a nichrome wire and a bimetal switch and the operation of the bimetal switch has been controlled by heating of the nichrome wire so that a pulse of high voltage may be generated by selfinduction of a current in a stabilizer choke coil, and such pulse has been used as the trigger for the discharge lamp. However, in the starting of the discharge lamp, the current flowing through the nichrome wire is so great that the arc produced between contacts when the bimetal switch is opened is great enough to fuse the contacts together in some cases. This may result in an excess current flowing through the stabilizer to heat and burn the stabilizer or the connecting wires. A greater problem is that since the current flowing through the nichrome wire is great as already noted, the pulse voltage induced in the stabilizer choke coil is extremely high and usually exceeds 10 KV. When a discharge lamp requiring a high starting voltage is to be started, a pulse of higher voltage than the starting voltage must be applied to the discharge lamp, but if the pulse voltage is higher than required voltage, there is the possibility of damaging the stabilizer or the connecting wires. It is therefore necessary to control the starting current flowing at the starting of the discharging lamp, as well as the pulse voltage generated by the starting current.
On the other hand, in the discharge lamp of the described type having a simple starting circuit consisting of a nichrome wire and a bimetal switch and contained within the outer envelope of the discharge lamp, a problem occurs as to how to accommodate the starting circuit in the internal space of the envelope so as not to intercept the light emitted from the light emitting tube. Further, whether the discharge lamp can be safely and reliably started or not becomes an important point.